1. Field of the Invention
The present invention relates to a magnetic carrier and a two-component developer that are used in electrophotographic systems, electrostatic recording systems, and electrostatic printing systems.
2. Description of the Related Art
The developing systems used in, for example, electrophotography, include monocomponent developing systems, which use only toner, and two-component developing systems, which use a mixture of toner with a magnetic carrier.
Two-component developing systems, because they use a magnetic carrier, have an excellent ability to triboelectrically charge the toner and offer the advantages over monocomponent developing systems of more stable charging characteristics and better long-term maintenance of a high image quality. In addition, two-component developing systems provide an excellent performance with regard to supplying toner to the developing zone and in particular are frequently used in, for example, high-speed copiers.
For example, a heavy metal-containing ferrite carrier has been used as this magnetic carrier. However, the high density and large saturation magnetization that occur in this case result in a rigid magnetic brush, and as a consequence deterioration of the developer, i.e., the generation of spent carrier and external additive deterioration for the toner, is prone to occur.
An Mg-type ferrite carrier with a low specific gravity has been introduced as a result. However, when the saturation magnetization of an Mg-type ferrite is increased, the resistance then declines and as a consequence it has been quite difficult to optimize both the magnetization and the resistance.
For example, an Mg-type ferrite carrier that substantially does not use heavy metal, including Mn, is provided in Japanese Patent Application Laid-open No. 2010-39368. Due to the use of a prescribed Ti content, this carrier exhibits a suitable unevenness in the carrier surface and achieves stable charging characteristics and longer life.
In addition, due to its use of a prescribed Mn content, the carrier provided in Japanese Patent Application Laid-open No. 2010-281892 strikes a balance between magnetization and resistance by controlling the grain structure within the ferrite core and can inhibit carrier scattering.
By stabilizing charging through an optimization of the magnetization and resistance, these proposals have provided excellent images when used in low-speed devices. However, the developing performance has been inadequate when used in high-speed devices (50 sheets/minute or more) and during durability testing the image density has undergone variation and/or blank dots have been produced.
Various carriers having a reduced specific gravity brought about by filling a resin into a porous ferrite core have also been proposed. The proposal is made in Japanese Patent Application Laid-open No. 2010-61120 that carrier adhesion can be inhibited—even during image printing at low image ratios—by using a carrier provided by controlling the pore diameter in ferrite core particles as measured by the mercury intrusion method. However, the amount of charge on the toner readily assumes excessive levels in high-speed devices and there is still room for improvement.